Ubuntu – I am looking for a small (Ubuntu) open-source-project for which I can code

Yes, you are supposed to create an Ubuntu Wiki page about yourself.

To do this, go to https://wiki.ubuntu.com/YourName (replacing YourName with your actual name) and click on Create a new empty page.

As for what information to put on your wiki, I would look at what existing Ubuntu Members have on their wiki pages.

There is a list of Ubuntu Members at https://launchpad.net/~ubuntumembers/+members#active. Click on some profiles and see their respective wiki pages.

I assume there is terminal emulator widget in your geany (which is suggested by geany package). Click on it (In lower-left corner) and first navigate to folder in which is your source file:

cd Path/To/Source

To compile file YourSourceFile.c type:

gcc YourSourceFile.c -o ProgramName

To run the program type:

./ProgramName